You're digging into a classic and important question regarding grounding electrode systems! Let's break it down

Multiple Metal Underground Water Pipes Directly Connected to the Earth

Yes, you are correct. If a building is supplied by multiple metal underground water pipes, each that meets the criteria of 250.52(A)(1) (metal, direct contact with the earth for 10 ft or more), then each must be connected to form part of the grounding electrode system. Rationale

The idea behind the code is to ensure that all available grounding electrodes are utilized to provide the lowest possible impedance path to earth for fault currents. Each qualifying metal water pipe can independently serve as a grounding electrode and must be included in the overall system to maximize its benefit.

Example (Your Fire Sprinkler/Domestic Water Scenario)

If you have two separate metal water laterals (one for domestic and one for fire suppression), and both run underground in direct contact with the earth for 10 feet or more, then both must be connected to the grounding electrode system.

Interconnected Metal Water Piping System

This is where it gets a bit more nuanced, and the interpretation hinges on a few factors: Complete Metal Piping System

If you have a continuous metal water piping system that connects the different underground sections within the building, then potentially it acts as a single, extended grounding electrode. However, the key here is "continuous." You need to ensure there aren't any isolating couplings or sections of non-metallic pipe that would interrupt the electrical continuity of the water piping system.

AHJ Interpretation

This scenario is often where the Authority Having Jurisdiction (AHJ) will have the final say. Some AHJs might consider the entire connected system as one grounding electrode, while others might require separate connections if the underground sections are significantly distanced from each other, even if they are interconnected by metal piping.

Bonding vs. Grounding

You will likely still need to bond the metal water piping system within the building even if you are not using it as a grounding electrode.

Example (Your Outbuilding Scenario)

You have a water service entering the main house, going to an outbuilding. All are metal and at least 10' of each are underground. The main point of concern is whether the outbuilding now needs to have its electrical system grounded to the continuous water pipe going back to the house. This is an interesting scenario that the NEC does not specifically address, but an AHJ may have an opinion. There's no reason why the outbuilding couldn't have its own electrode, even if connected to the house's metal water pipe. However, an AHJ may disagree, and the outbuilding would need to be bonded to the house's grounding electrode system.

Important Considerations & Best Practices 250.53(D)

The connection to the metal water pipe must be made within 5 feet of where the water pipe enters the building.

250.68(C)(1)

The grounding electrode conductor must be sized according to Table 250.66 (based on the size of the ungrounded service conductors).

Supplemental Electrode

You must supplement the metal water pipe electrode with an additional electrode (e.g., ground rod, ground ring, concrete-encased electrode) as per 250.53(A). The metal water pipe alone is not sufficient.

Corrosion Concerns

Galvanic corrosion can be a concern when dissimilar metals are connected in a grounding system. Using appropriate connectors and taking steps to mitigate corrosion is important.

Documentation

It's crucial to thoroughly document your grounding electrode system, including the location of all electrodes and connections. This is especially important if there are multiple electrodes or complex interconnections.

In summary: If you have multiple independent metal underground water pipes that each meet the 10 ft/direct contact with earth requirement, each must be connected to the grounding electrode system. If the water pipes are interconnected within the building with a continuous metal piping system, the entire system may be considered a single grounding electrode, but this is subject to AHJ interpretation. Always supplement a metal water pipe electrode with another type of grounding electrode. Always verify your interpretations with the AHJ and get their approval for your specific installation. Remember the 5' rule, sizing requirements of 250.66, and the need for supplemental electrodes.Consulting with a qualified electrical engineer or inspector is highly recommended for complex or unusual situations. They can provide site-specific guidance and ensure compliance with all applicable codes and regulations. Flag for review